Zinc plating



ating characteristics.

Patented Mar. 31, 1942 ZINC PLATING Clayton F. Ruebensaal, Cleveland,Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington,Del., a corporation of Delaware No Drawing.

Application October.19, 1938, Serial No. 235,782

3 Claims. (01. 204-55) This invention relates to the electrodepositionof zinc from sodium zincate solutions and is more particularly directedto the use of glycollic acid or its salts as an addition zincate platingbaths.

. It is frequently the practice to produce zinc electrodeposits. fromacid baths which are made up. for instance, with sulfuric acid. Thoughthese acid baths are relatively inexpensive, they require specialacid-resisting equipment and they have a relatively poor throwing power.

Zinc is plated with great success from alkaline baths containingcyanide. Deposits of excellent character are obtained from thesecyanide-zinc plating baths and the baths are themselves relaagent forsodium therefore, that glycollic acid may be added initively much lesscorrosive than the acid baths.

Cyanide-zinc plating baths have excellent'opercyanide-zinc plating bathsare relatively expensive. Alkaline zinc baths omitting cyanide are quiteinexpensive but they have not heretofore proven satisfactory. Thesealkaline zinc baths are com- Unfortunately, however,

posed of sodium zincate and are customarily pre.-.

pared by dissolving zinc oxide in sodium hydroxide. The sodium zincatebaths are less expensive than even the acid baths heretofore used, andthey have a relatively great throwing power. Sodium zincate baths arerelatively much less corrosive than acid baths, and they maybe handledin ordinary iron tanks.

Unfortunately, the deposits obtained from sodium zincate baths arespongy and are of .ex-

ceedingly poor character. Addition agents such as have been foundvaluable in improving the character of deposit in cyanide-zinc platingbaths are without substantial effect or are actually deleterious insodium zincate baths.

- I have found that by the use of glycollic acid (CH2OHCOOH) or of itssalts as an addition agent, sodium zincate baths may be markedlyimproved and deposits of improved character may be obtained. Glycollicacid and glycolates increase the range of current densities over which asmooth and satisfactory deposit may be obtained, and the maximumpermissible current density is considerably increased. Glycollic acidand its salts are apparently quite stable and no harmful decomposition.products are formed either upon standing of the bath or uponelectrolysis.

The amount of glycollic acid or glycolate to employ will depend to someextent upon the exact character of the sodium zincate bath and upon theeffect desired. In general, the optimum amount to use in a particularinstance can best be determined by a few simple trials. In general Ihave found that from about seven to thirty grams per liter of glycollicacid, or of a glycolate, is satisfactory while more particularly I havefound that around twenty-three grams per .liter is about the optimumamount of glycollic acid for a number of specific in stances. I

It will be understood that when glycollic acid is added to a sodiumzincate bath it is likely that sodium glycolate forms and it'is thisreaction product which is actually present as an active addi ion agent.It will be understood,

tially in the. form of sodium glycolate, and it will also be understoodthat other soluble glyco-'- lates such as the. potassium, ammonium andZinc salts of glycollic acid may be used.

Glycollic acid or one of its salts may :conveniently be added to asodium zincate bath in the amount required. The bath composition may bemaintained by adding a'small additional amount of glycollic acid orglycolate from time to time. The addition agent may, if desired, be

incorporated in a prepared composition including anylor all of the bathingredients so that upon dissolution-in water a sodium zincate bath'will be formed.

It will be-understood that glycollic acid or its salts may be usedaccording to my invention with any sodium zincate bath, and that suchbaths may contain additional addition agents, brighteners, and otherbath-modifying constituents of a character known to the art.

While I have mentioned specific illustrative conditions in theforegoing, it will be understood that those skilled in the art mayemploy glycollic acid or its salts as addition agents for sodium zincatebaths in. any desired manner without departing from the spirit of myinvention. i I claim:

1. In a process for the electrodeposition of zinc from a sodium zincatebath, the step comprising depositing zinc from an electrolyte comprisedessentially of sodium zincate and free of cyanide, deposition beingeffected in the presence of from about 7 to 30 gramsper liter of asubstance 3.'A plating composition comprising sodium zincate and a minoramount of a substance selected from the group consisting of glycollicacid and its salts, the composition being free of cya nide. r

CLAYTONF. RUEBENSAAL.

